1. Technical Field
This invention pertains generally to the mounting and connecting of electronic devices, and more particularly, to a method of making an improved solder ball mounting pad on a substrate.
2. Related Art
An increasing demand for electronic equipment that is smaller, lighter, and more compact has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting "footprints."
One response to this demand has been the development of the so-called "flip-chip" method of attachment and connection of semiconductor chips to substrates. Sometimes referred to as the "Controlled Collapse Chip Connection," or "C4," method, the technique involves forming balls of a conductive metal, e.g., solder or gold, on input/output connection pads on the active surface of the chip, then inverting, or "flipping" the chip upside-down, and "reflowing" the conductive balls, i.e., heating them to the melting point, to fuse them to corresponding connection pads on a substrate.
Another response has been the development of so-called ball grid array ("BGA") semiconductor packages that "surface mount" and electrically connect to an associated substrate, e.g., a printed circuit board ("PCB"), with a plurality of solder balls in a method, sometimes referred to as the "C5" method, that is analogous to the flip-chip method described above for mounting and connecting dies.
In both the C4 die and C5 package mounting and connection methods, a plurality of solder balls are attached to respective solder ball mounting lands, or pads, defined on a surface of the die or package. The solder ball mounting pad may, but need not be, defined by an opening in an insulative layer or mask called a "passivation layer" in the case of a semiconductor die, or a "solder mask" in the case of a BGA package, as described below.
FIG. 1A is a top plan view of a portion of a substrate 10 having a solder-mask-defined ("SMD") solder ball mounting pad 28 formed thereon in accordance with the prior art. FIG. 1B is a cross-sectional view looking into the substrate 10 and pad 28 along the lines IB--IB in FIG. 1A. The substrate 10 may comprise a sheet 12 of an insulative material, such as fiberglass, polyimide tape, or ceramic, or alternatively, it may comprise a semiconductor chip or die.
The pad 28 typically comprises a layer of metal, e.g., copper, aluminum, gold, silver, nickel, tin, platinum, or a combination of the foregoing that has been laminated and/or plated on a surface of the sheet 12, then patterned using known photolithography techniques into a central pad structure 14, which may include one or more circuit traces 16 (shown by dotted lines) radiating outward from it. Alternatively, or in addition to the traces 16, a plated-through hole, called a "via" 18 (shown by dotted lines), may connect the central pad 14 with the opposite surface of the sheet 12.
An insulative mask 20, referred to as a passivation layer in the case of a semiconductor die, or a solder mask in the case of a BGA package, is formed over the metal layer, including the central pad 14. The insulative layer 20 may comprise an acrylic or a polyimide plastic, or alternatively an epoxy resin, that is silk screened or photo-deposited on the sheet 12. An opening 22 is formed in the insulative mask 20 to expose a central portion 28 of the central pad 14, and a solder ball 24 (shown dotted in FIG. 1A) is attached to the pad 28 thus exposed. Since the mask 20 prevents the solder of the solder ball 24 from attaching to any portion of the central pad 14 other than the portion 28 that is exposed through the opening 22, the pad 28 is referred to a solder-mask-defined or SMD-type of solder ball mounting pad, as above.
A non-solder-mask-defined ("NSMD") solder ball mounting pad 28 in accordance with the prior art is illustrated in the plan view of FIG. 2A, wherein features similar to those in the SMD pad 28 of FIG. 1A are numbered similarly. FIG. 2B is a cross-sectional view looking into the substrate 10 and pad 28 along the section lines IIB--IIB in FIG. 2A.
As may be seen from a comparison of the two sets of figures, the respective pads 28 are very similar, the exception being the size of the opening 22 in the insulative mask 20. In particular, in the NSMD pad 28 of FIGS. 2A and 2B, the opening 22 exposes the entire central pad 14, along with a portion of the surface of the sheet 12 and a portion of the optional circuit trace 16, such that the molten solder of the solder ball 24 can wet and attach to not only the entire upper surface of the central pad 14, but also to the vertical side walls 26 of the pad and the circuit trace.
While each of the SMD and the NSMD prior art solder ball mounting pads 28 has some advantages associated with it, each also has some disadvantages, as well. The SMD pad 28 shown in FIGS. 1A and 1B is the most commonly used solder ball mounting pad today. It provides good "end-of-line" (i.e., at the end of the semiconductor package fabrication line) ball 24 shear resistance because, as may be seen in FIG. 1A, the insulative mask 20 overlaps the entire peripheral edge of the central pad 14, and hence, resists ripping of the pad from the sheet 12 when mechanical forces act on the solder ball 24 attached thereto. However, as may be seen in FIG. 2B, the insulative mask 20 covers no part of the central pad 14 portion of the NSMD pad 28, and consequently, the latter has a relatively lower end-of-line ball 24 shear resistance.
The SMD pad 28 also affords relatively better control of the "x-y" positional tolerances of the solder ball 24, i.e., better control of the lateral position of the solder ball 24 on the surface of the sheet 12, than does an NSMD pad 28 having one or more circuit traces 16 leading from it, such as the one shown in FIG. 2A. This is because the x-y position of the ball 24 on the sheet 12 is affected by two positional parameters: 1) the position on the sheet 12 of the centroid of the opening 22 in the insulative mask 20, and 2) the position on the sheet of the centroid of the area of metal 28 exposed by the opening in the mask, i.e., the area wetted by the molten solder of the ball 24 when the latter is attached to the pad 28. In both instances, the center of gravity ("C.G.") of the solder ball 24 tends to align itself over each of the two respective centroids. As a result, when the centroid of the opening 22 does not coincide with the centroid of the area of exposed metal 28, the C.G. of the solder ball 24 will be positioned approximately half way along a line extending between the two centroids.
As may be seen in FIG. 1A, the shape, or "pattern," of the area of the SMD pad 28 exposed by the circular opening 22 in the insulative mask 20 is, by definition, also circular, and hence, radially symmetrical about the centroid of the exposed area of the pad. Also by definition, the centroid of the pad 28 coincides with the centroid, viz., the center, of the circular opening 22. Hence, so long as the opening 22 in the insulative mask 20 is located within the boundary of the central pad 14, the x-y tolerances on the ball 24 will depend only on the x-y positional tolerances on the centroid of the opening 22, and not on the x-y positional tolerances of the centroid of the pad 14. The presence of the optional via 18 will not change that result, provided the latter is also centered in the opening 22.
However, as may be seen in FIG. 2A, the shape of the NSMD pad 28 exposed by the opening 22 in the mask 20, which includes the entire central pad 14, as well as a portion of the circuit trace 16, is only bilaterally symmetrical about a line passing through the center of the central pad and the circuit trace. Consequently, the centroid of the NSMD pad 28, i.e., of the exposed area of metal, is shifted slightly toward the circuit trace 16, and away from the centroid of the opening 22, which is typically centered on the central pad 14. Hence, the C.G. of the solder ball 24 will be positioned about half way between the respective centroids of the opening 22 and the NSMD pad 28
Thus, the x-y positional tolerances on the ball 24 on an NSMD pad 28 will depend not only on the x-y positional tolerances of the centroid of the opening 22, but also the x-y positional tolerances of the centroid of the NSMD pad 28 as well. The presence of the optional via 18 will not change that result, even if the latter is centered in the opening 22. Moreover, even without a circuit trace 16 or via 18, misalignment of the solder ball 24 can still occur in an NSMD pad 28 if the centroid of the pad 28 is not coincident with the centroid of the opening 22.
While the x-y positional misalignment of the ball 24 relative to the opening 22 resulting from this C.G. "shift" is relatively small, it should be understood that a C4-mounted die or a C-5-mounted semiconductor package can typically have a large number, e.g., up to nine hundred, of such balls on its mounting surface, and that accordingly, these slight misalignments in the array of balls can be additive, such that in some cases, the die or package cannot be successfully mounted to an associated mounting surface.
The prior art NSMD-type pad 28 shown in FIGS. 2A and 2B is used less frequently today than the SMD-type of pad 28 shown in FIGS. 1A and 1B. However, the NSMD pad does have some advantages over the SMD pad. For example, as may be seen from a comparison of FIGS. 1B and 2B, in the NSMD pad 28 in FIG. 2B, the solder of the ball 24 wets down and attaches to the vertical side walls 26 of the central pad 14 and the circuit trace(s) 16, if any, to form a fillet 30 around their respective peripheries. This fillet structure 30 helps to distribute stresses resulting from thermal aging so that the stresses do not concentrate at the interface between the pad 28 and the ball 24. As shown in the SMD pad 14 of FIG. 1B, however, the interface between the pad 14 and the ball 24 lacks this structure, and instead, consists of a simple interface between two planar surfaces.
Another area of superiority of the NSMD pad 28 involves the related problem of "gold embrittlement." It is a common practice in the industry to plate solder ball mounting pads 28 with a layer of nickel, followed by a layer of gold, to improve the solderability of the pads. During the attachment of the balls 24 to the pads 28, some of the tin in the solder combines with the gold to form a brittle "intermetallic" compound of gold and tin, which breaks away from the solder-gold interface and floats up into the molten solder ball 24. Some of the tin in the solder also migrates through the gold to combine with the nickel and form another brittle intermetallic compound of nickel and tin.
The joint strength between the solder ball 24 and the gold-nickel plated pad 28 is good immediately after the attachment of the ball. However, with thermal aging at an elevated temperature, as during a component "burn-in" procedure, some of the tin-gold intermetallic compound diffuses back into contact with the tin-nickel intermetallic compound at the joint, and the two combine to form a hard, brittle interface that is very susceptible to stress-induced cracking.
However, as discussed above, the fillet structure 30 in the NSMD pad 28 of FIG. 2B acts to distribute the stresses associated with thermal aging so that the stresses do not concentrate at the interface between the ball 24 and the pad 28, and hence, at the interface between the two intermetallic compounds. Therefore, the NSMD pad 28 of FIG. 2B exhibits a resistance to the gold embrittlement phenomenon that is superior to the SMD pad 28 of FIG. 1B, because the latter pad lacks this stress de-concentrating structure.
Thus, while the SMD pad 28 has greater end-of-line ball 24 shear resistance and provides better ball x-y positional tolerances than does the NSMD pad 28, the NSMD pad has superior thermal cycle reliability and resistance to gold embrittlement. In view of the foregoing, it would be very desirable if a method could be discovered for making solder ball mounting pads 28 on a substrate 10 that combined the advantages of both types of prior art pads and eliminated some of their disadvantages.